Cathode-ray tubes (CRTs) have widely been recognized as important display devices for visually displaying images, waveforms, graphs, characters, etc. Fast writing speed and brighter image intensity are two important features of CRTs over other two dimensional display devices. CRTs are especially key display devices for oscilloscopes.
CRTs comprise a neck portion including an electron gun and deflection plates, a faceplate portion on the inner surface of which a phosphor screen is provided, and a funnel portion coupling the foregoing two portions. As shown in FIG. 1, an electron gun in the neck portion 12 of CRT 10 comprises cathode 14, first control grid 16, second grid 18, focus electrode 20 and astigmatism electrode 22. For indirect heating type CRTs, heater electrode 15 is used to heat the oxide layer of cathode 14 to emit thermionic electrons therefrom.
Operation potentials to various electrodes of the electron gun are typically as follows: A constant negative high voltage (-HV) of about -3 kV is normally applied to cathode 14. A floating DC voltage source 24 elevated to the cathode potential (-HV) is connected to heater 15 to heat up cathode 14. Connected to control grid 16 is another high voltage source 26 floated by the output from Z-axis amplifier 28 which controls the beam intensity during sweep periods, blanking/unblanking depending on sweep/sweep retrace periods, and chop blanking. Neon tubes 30a and 30b are connected between control electrode 16 and cathode 14 to protect the triode section of the electron gun from excessive voltage. Although not shown, a constant voltage is applied to second grid 18. The focus voltage is controlled by a voltage divider including focus control potentiometer 32 and a few other resistors connected in a string between cathode 14 and a reference potential source. Finally, a controllable voltage at the wiper arm of astigmatism control potentiometer 34 is applied to astigmatism electrode 22.
The electron beam emitted from cathode 14 is well focused on faceplate 11 by controlling focus control potentiometer 32 and astigmatism control potentiometer 34. Optimum focus and astigmatism voltages are a function of the control grid bias voltage and potentiometers 32 and 34 are varied as the grid bias voltage is changed so as to increase or decrease the beam current. Hitherto, such focus and astigmatism voltages were controlled independently so that the electron beam is well focused on the phosphor screen. Such readjustment is time consuming and impractical, especially when the beam current or the grid bias voltage is varied to different levels in synchronism with vertical channel switching or horizontal sweep period of a multichannel, alternate sweep oscilloscope.